
Yes Maythan was the Fate Stone. Carsen had looked in the royal library as a child for a way to get rid of his curse and read about the stone. He ripped the book out of frustration that it was just a fairy tale and says "There's no way something like the Fate Stone could ever exist". It's in Ch 71.
It was right under CARSEN'S nose as it was inside Maythan all along. And he's holding it now because Maythan has vanished thus revealing the stone.
There is literally no part where anyone physically holds the Fate Stone until now. It says about it roaming the earth and changing fate which makes sense if it is inside Maythan because then it can 'roam' because it's moving with Maythan and Maythan has been the one changing the storyline/fates around him. The Fate Stone brought him to the world and that's why Maythan could never find it because it was inside him, likely also giving him that power he has to nullify magic.

I don't know I never read the book.
When I got to chapter 71 I had also guessed he was the stone just based on the prophecy, context in the story, and general story structures.
I thought it was pretty clear in this chapter as well. I think you misremembering what happened before in the story made it more difficult to connect why it was there since you thought Carsen had it the whole time. Thinking it had already been found would change how you view the chapter when you initially read it.
So Maythan was the Fate Stone all along?? I mean the monologue was kinda suggesting it
"roamed around the kingdom containing the power to change one's ultimate fate" -Carsen's death
"it could be right next to you" -Maythan prolly